Currently, there are two major techniques to utilize solar energy; photovoltaics (PV) that directly convert light into electricity, and solar thermal energy that heats working fluids to run a turbine or to provide domestic hot water. Although solar thermal energy can generate both electricity and heat, applications have been limited to utility-scale power generation due to expensive parts and high installation costs.
The core of a concentrated solar power (CSP) system is the solar receiver. Current designs for CSP systems are focused on industrial scale operation, leaving residential solar systems to only use of photovoltaic panels. Photovoltaic panels are an inefficient form of solar power because they only utilize a small portion of the solar spectrum and they operate at low conversion efficiencies that are worsened by heat. CSP systems on a residential scale provide thermal energy, which can be used for a variety of purposes including hot water, space heating, and absorption refrigeration.
CSP has historically been the most cost effective method of harnessing solar energy for electricity production on the utility scale; solar energy is reflected using mirrors and concentrated into a receiver to heat a working fluid that runs a turbine. CSP also possesses a distinctive potential for combined cooling, heating, and power (CCHP) generation systems due to the high temperature of rejected heat. Waste heat present after electricity generation can be further extracted for space heating, domestic hot water, and absorption chiller refrigeration. Energy systems with CCHP have a much higher energy conversion efficiency compared to other technologies. Up to 75% of incoming solar radiation can be converted into useful energy including electricity. Despite the benefits of CSP, the cost per energy produced is not currently competitive at the residential scale, since a small mechanical system has a large ratio of parasitic heat loss to power production and high component/installation costs per power output.
Although utility-scale CSP has developed and shown success over the past few decades, small-scale CSP for distributed combined heat and electricity has not developed mainly due to economic challenges. CSP requires a solar tracker, which alone constitutes up to 40% of the installation cost in CSP. Moreover, existing systems generate electricity mostly through the use of turbines, which are inefficient and expensive at the small scale. Turbines also cause noise and require regular maintenance.
Thermoelectric materials directly convert heat into electricity at a solid state, making them reliable, easily scalable, and free of vibration or noise. Thermoelectric systems have been used as the power systems for many deep space missions for more than 30 years. Unlike photovoltaics (PV), their energy conversion efficiency does not degrade with increasing temperature and the entire spectrum of solar energy in the form of heat can be utilized. Thermoelectric modules show higher power to area/weight ratios and generate more power under cloudy conditions than PV cells. The major drawback of this technology is low efficiency.
With currently available materials, energy conversion efficiency is barely 10% under a temperature difference of 200K. Despite low efficiency, thermoelectric materials have the advantage of generating electricity from any temperature difference. Most significantly, they can generate electricity under small temperature differences, where the use of other energy conversion technologies cannot be economically justified. For combined heat and electricity, low conversion efficiency has less adverse effects, since rejected heat can be further extracted for other purposes. At the residential scale, most of electricity-intense appliances are related to heating or cooling, and can be replaced with thermal energy using absorption chillers. Hence, the thermoelectric system is an economically viable solution for the residential scale combined heat and electricity from concentrated solar power. Currently, most research efforts have been focused on the development of highly efficient thermoelectric materials. Consequently applications have only been restricted to niche markets, such as space missions or car seat cooling/heating.
Because of the high installation and fixed costs of existing solar trackers, the U.S. Department of Energy (DOE) has challenged researchers to reduce the installation cost of heliostat fields from $200 per square meter to $70 to lower total capital costs on utility-scale power plants by 25%. High fixed costs exist for conventional heliostats because of the robust steel supports needed for stability. Specifically, the two main cost drivers beside installation in large heliostat fields are the drive motor assemblies and the mirror support/structure/foundation. The installation of this heavy hardware is the other major component of tracker assembly cost and requires an automated assembly and deployment system.
The predominant two-axis tracker design is commonly termed a mast tracker. Mast trackers require a very stout pole, or “mast”, to be drilled deep into the ground to support normal loading. The mast height is at least one half of the panel height above the ground so that the tracker can orient toward the sun at low elevation angles. The requisite foundations result in considerable geological concerns for site planning, as well as heavy machinery, contributing to the installation costs. Since existing two-axis trackers control their load from a single central point, the drive assemblies must be very heavy duty and added trusses are often needed to keep the structure from flexing or sagging at the extremes. Several two-axis mast trackers have been designed with reduced installation costs as a motivating factor. The PVT 7.2DX, manufactured by PV Trackers, utilizes a tripod structure for support. The system is compatible with helical piles reducing installation costs by eliminating the need for concrete foundations. The Google RE<C initiative designed a two-axis heliostat, also with a triangular base, and ground securing is accomplished with a single helical pile. Finally, the Opel SF-45 is a utility-grade tracker for use with PV panels. The manufacturer claims it can be installed by a two person team without any field welding. Qbotix has developed a system, which allows an entire array of PV panels to be adjusted discretely by a single robot that runs on a track from panel to panel. While this can effectively cut costs by using a single actuation unit for multiple panels, the discrete nature cannot be used for solar thermal applications. In all cases above, the load is carried atop a tall mast and requires heavier materials to achieve the rigidity necessary to ensure acceptable pointing accuracy.
What is needed is a device that provides heat and electricity to a single-family home from concentrated solar energy using thermoelectric modules.